Method and apparatus for increasing the quality of the receiver synchronization of QAM or CAP modulated modem connection

ABSTRACT

The invention relates to a method and apparatus for improving the quality of receiver synchronization on QAM- or CAP-modulated modem connections having an adaptive linear equalizer. The invention is based on the finding that in a QAM- or CAP-modulated data transmission system where the symbol rate and the average carrier frequency are in fixed relationship with each other, the vector formed by the In-phase and the Quadrature component of the detector input signal rotates with the timing phase shift of the receiver. The quality of receiver synchronization is improved by utilizing information extracted from the vector rotation.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/FI01/01060 which has an Internationalfiling date of Dec. 5, 2001, which designated the United States ofAmerica.

FIELD OF THE INVENTION

The invention relates to a method and an apparatus for improving qualityof receiver synchronization on a modem connection that utilizes QAM- orCAP-modulation (Quadrature Amplitude Modulation, Carrierless Amplitudeand Phase).

BACKGROUND OF THE INVENTION

In the text describing the prior art and the features of the presentinvention, the following abbreviations are used:

CAP Carrierless amplitude and phase modulation FIR Finite impulseresponse (filter) QAM Quadrature amplitude modulation RX Receiver TXTransmitter

On a modem connection, the operation of the receiving device (receiverRX) must be synchronized with the operation of the transmitting device(transmitter TX). The transmitter converts the digital bit stream to betransmitted into a symbol stream that is then converted into an analogsignal for transmission over a copper wireline. The received analogsignal is processed by analog signal processing techniques, where-uponthe signal is converted back into a digital sample stream. In thereceiver, the stream of symbol values thus passed over the transmissionchannel is regenerated from these sample values by digital signalprocessing techniques. Finally, the original bit stream is regeneratedfrom the symbol values extracted in the receiver. Receiversynchronization is implemented by controlling the clock of the receiverso that the instants of symbol decisions are as closely as possible at aconstant phase in regard to the phase of the symbols sent by thetransmitter. The timing of receiver operation is controlled by a signalwhose value is dependent on the phase difference between the decisioninstants and symbol stream received over the transmission channel.Hereinafter, the technique of generating such a timing signal that isproportional to the phase difference is called a synchronization methodand the respective phase difference is called the timing phase ofsynchronization. To the quality of data transmission, it is essentialthat the timing phase varies as little as possible over time.

DESCRIPTION OF PRIOR ART

Traditional synchronization techniques herein are square synchronizationand correlation synchronization, both of which are described in greaterdetail, e.g., by E. A. Lee and D. G. Messerschmitt in DigitalCommunication (Chapter 17—Timing Recovery), Kluwer Academic Publishers1994. A problem hampering these methods is that the timing controlsignal is corrupted with noise that arises from the generation processof the control signal. Such noise causes variations in the timing phasethus deteriorating the quality of data transmission. Noise can beattenuated by narrowing the passband of the low-pass filter that is anintegral part of the receiver means, but this approach in turndeteriorates the capability of the synchronization method to followchanges in the data rate of the symbol stream and to compensate for theeffects of possible disturbances imposed on the analog circuitry of thereceiver.

Receiver synchronization with advantageous noise characteristics, yetbeing fast enough to follow data rate variations of the transmittedsymbol stream, can be realized by utilizing information carried alongwith the detector input signal, see e.g. Yasuharu Yoshida, U.S. Pat. No.4,528,512. The timing control signal can be formed on the basis of thedetector input signal only if the detector of the system is not precededby any other signal processing mechanisms that are capable ofcompensating for timing phase shifts. An example of these mechanisms isan adaptive FIR equalizer that in practice must be used to compensatefor transmission channel distortion on a modem connection utilizing QAMor CAP modulation. The linear equalizer must be adjusted not only at theinitialization of a connection, but also during ongoing datatransmission, because variations in ambient conditions such as channeltemperature cause changes in the channel distortion. Using the detectorinput signal information alone in conjunction with an adaptive linearequalizer would eventually cause the timing phase and the distributionof the equalizer tap coefficients to drift continually in oppositedirections so as to mutually compensate for the drift in either one ofthese adjustments. This interaction is due to the fact that the timingcontrol signal derived from the detector input signal is not independentof the distribution of equalizer tap coefficients. The situation wouldfinally wind up into a collapse of the data transmission connection whenthe distribution of equalizer tap coefficients ultimately drifts outfrom the temporal length of the linear equalizer.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the drawbacks ofthe above-described techniques and to provide an entirely novel type ofmethod and apparatus for use on a digital communications channel.

It is an object of the invention to provide a method and apparatussuited for improving the capability of receiver synchronization used onQAM- or CAP-modulated (Quadrature Amplitude Modulation, CarrierlessAmplitude and Phase) modem connections equipped with an adaptive linearequalizer to cope with changes in the symbol stream rate and compensatefor the effects of possible disturbances interfering with the operationof the system's analog parts without the risk of impairing the noiseproperties of clock synchronization. The applications of the inventionare focused on systems having a fixed relationship between the symbolrate and the average carrier frequency of the modulated signal.

The goal of the invention is achieved by implementing receiversynchronization control through firstly using information that isproportional to the rotation of the vector formed by the In-phase andQuadrature components (d_(I) and d_(Q)) of the detector input signaland, secondly, an additional signal that is proportional to the timingphase and is obtained via a method whose output signal about theequilibrium point of the timing phase is independent from the state ofthose signal processing mechanisms that are capable of compensating forthe drift of the timing phase.

The present invention provides a method for improving quality ofreceiver symbol synchronization on a digital communication connection,in which digital communication connection:

-   -   a signal passed over a channel is received, the received signal        being modulated according to one of the following line codes:        quadrature amplitude modulation and carrierless amplitude and        phase modulation,    -   distortion caused by the channel to the received signal is        compensated with an adaptive linear equalizer, and    -   symbols are recovered from the received signal with a detector.

The method according to the present invention comprises:

-   -   generating a control signal that is proportional to rotation of        a vector formed by an In-phase component and a Quadrature        component of an input signal of the detector,    -   generating an additional signal that is proportional to a timing        phase of the receiver symbol synchronization and, in the        vicinity of an equilibrium point of the timing phase, said        additional signal is independent from state of signal processing        mechanisms that can compensate a drift of the timing phase, and    -   controlling the receiver symbol synchronization on the basis of        a combination of said control signal and said additional signal.

The present invention provides an apparatus for improving quality ofreceiver symbol synchronization on a digital communication connection, areceiver of the digital communication connection comprising:

-   -   means for receiving a signal passed over a channel, the received        signal being modulated with one of the following line codes:        quadrature amplitude modulation and carrierless amplitude and        phase modulation,    -   an adaptive linear equalizer for compensating distortion caused        by the channel to the received signal, and    -   a detector for recovering symbols from the received signal.

The apparatus according to the present invention comprises in thereceiver of the digital communication connection:

-   -   means for generating a control signal that is proportional to        rotation of a vector formed by an In-phase component and a        Quadrature component of an input signal of the detector,    -   means for generating an additional signal that is proportional        to a timing phase of the receiver symbol synchronization and, in        the vicinity of an equilibrium point of the timing phase, said        additional signal is independent from state of signal processing        mechanisms that can compensate a drift of the timing phase, and    -   means for controlling the receiver symbol synchronization on the        basis of a combination of said control signal and said        additional signal.

The invention provides significant benefits:

-   -   The noise content of the timing phase rotation information is        minimal thus permitting a rapid control of phase lock without        deteriorating the quality of data transmission, whereby the        system becomes more able to adapt to changes in symbol rate and        to compensate for the effect of possible disturbances coupled to        the analog parts of the system.    -   The function of the additional timing control signal formed by        means of, e.g., the square or correlation method, is only to        prevent the above-mentioned phase drift that takes place at a        slow pace. Hence, the low-pass filtration used in such a        synchronization method based on, e.g., square or correlation        techniques, can have a substantially narrower bandwidth than        what would be required without the use of the additional timing        control signal extracted from the timing phase rotation        information. Resultingly, the quality of data transmission is        improved, since the noise contained in the timing control signal        is reduced by the narrower bandwidth of low-pass filtration.

The benefit of the control signal extracted from the rotationinformation is that it limits the rate-of-change of the timing phase sothat the adaptive linear equalizer can follow changes in the timingphase in an improved manner. Thus, the impairing effect on the qualityof data transmission due to noise in the additional timing controlsignal obtained by means of, e.g., the square or correlation method, isreduced in a substantial manner.

BRIEF DESCRIPTION OF FIGURES

In the following, the invention is described in more detail withreference to exemplifying embodiments elucidated in the appendeddrawings in which

FIG. 1 shows the detector input signal in an x,y coordinate system of asystem utilizing QAM or CAP modulation when a) the timing phase isoptimal and b) the timing phase has drifted away from the optimal timingsituation; and

FIG. 2 shows a receiver according to the invention utilizing QAM or CAPmodulation, whereby it must be understood that the receiver may alsocontain such signal processing mechanisms that are not drawn in thediagram, e.g. a DFE (Decision Feedback Equalizer).

DETAILED DESCRIPTION OF THE INVENTION

The theoretical background of the invention is explained in thefollowing treatise.

In a system utilizing QAM or CAP modulation and having the symbol rateand the average clock rate in a fixed relationship with each other, thevector (I, Q) formed by the I (In-phase) and Q (Quadrature) componentsof the detector input signal rotates when the a shift occurs in thetiming phase. The situation is elucidated in FIG. 1 for the 16-QAM or16-CAP modulation scheme. The relationship between the timing anglerotation φ [rad] and the shift of the timing phase τ [s] is:φ=ω_(c)τ,  (1)

where ω_(c)[rad/s] is the average carrier frequency of the QAM or CAPmodulation. The noise energy of the timing phase control signalextracted from the timing angle rotation is very small as compared to,e.g., the noise energy of a control signal obtained by the square orcorrelation method, for instance.

The timing control signal cannot be generated on the basis of therotation information of the timing angle alone, because the operationbecomes unstable in conjunction with an adaptive linear equalizer due tothe reasons described above.

An example of a system according to the invention is shown in FIG. 2.Herein, a signal transmitted over a data transfer channel 1 is firstconditioned by analog filters 2, whereupon the signal is subjected to ananalog-to-digital conversion performed with an analog-to-digitalconverter 3. Thereupon, if the quadrature amplitude modulation (QAM) isused, the signal is demodulated with a QAM-demodulator 4. The analogfilters 2, the analog-to-digital converter 3, and if the quadratureamplitude modulation (QAM) is used, also the QAM-demodulator 4constitute means for receiving the signal passed over the channel 1,wherein the received signal is modulated with one of the following linecodes: the quadrature amplitude modulation (QAM) and the carrierlessamplitude and phase modulation (CAP). Next, the signal path includes anadaptive linear equalizer 5 followed by a symbol detector 6. In the nextblock 7 takes place the decoding of symbols into a bit stream. Thetiming angle rotation information is formed in a block 25 from the inputand output signals of the detector 6. The block 25 constitutes means forgenerating a control signal 22 that is proportional to rotation of avector formed by an In-phase component and a Quadrature component of theinput signal of the detector 6. A timing control signal 21 is formed bycombining the control signal 22 generated on the basis of the timingphase rotation information with such an additional signal 23 formed in ablock 8 that is proportional to the timing phase and is formed so thatthis additional signal in the vicinity of the equilibrium point of thecorrect timing phase is independent from the state of those signalprocessing mechanisms that can affect the compensation of the drift ofthe timing phase. The block 8 constitutes means for generating theadditional signal 23 that is proportional to the timing phase of thereceiver symbol synchronization and, in the vicinity of an equilibriumpoint of the timing phase, the additional signal 23 is independent fromstate of signal processing mechanisms that can compensate a drift of thetiming phase. The additional signal 23 can be formed by means of, e.g.,a square or correlation method. In the exemplifying embodiment, block 8forming the additional signal takes its input signal from the inputsignal of adaptive linear equalizer 5. The timing control signal 21 isapplied to the control input of clock generator 9.

The combination of the control signal 22, which is obtained from thetiming phase rotation information, with the additional signal 23obtained using, e.g., a square or correlation method, may in practicetake place, e.g., in the following manner:timing control signal=coeff. 1×control signal obtained from rotationinformation+coeff. 2×additional signal,whereby the timing control signal is the sum of the output signals oftwo P-type controllers (proportional control). Coefficient 1 andcoefficient 2 are parameters selected so as to optimize the stabilityand quality parameters of the clock synchronization process. Block 24constitutes means for controlling the receiver symbol synchronization onthe basis of a combination of the control signal 22 and the additionalsignal 23. Furthermore, even more complicated controller constructionscan be used for generating the timing control signal. In practicalimplementations it has been found advantageous to use a techniquewherein the control signal extracted from the timing phase rotation istaken to a P-type controller while the additional control signal istaken to a PI-type (Proportional and Integrating) controller and theoutputs of these two are then summed. The generation block 25 of thecontrol signal obtainable from the timing angle rotation information canbe implemented, e.g., by computing an approximate value of the rotationangle as a vector product of the detector input signal vector with thesymbol decision vector:φ˜S_(I)d_(Q)−S_(Q)d_(I),  (2)where S_(I) and S_(Q) are the In-phase and Quadrature components ofsignal decision, while d_(I) and d_(Q) denote the In-phase andQuadrature components of the detector input signal. The same result isalso obtained by forming the vector product of the detector error vectorwith the symbol decision vector:

1. A method for improving quality of receiver symbol synchronization on a digital communication connection, in which: a signal modulated according to one of the following line codes: quadrature amplitude modulation and carrierless amplitude and phase modulation, is passed over the digital communication connection and is received, distortion caused by the digital communication connection to the received signal is compensated with an adaptive linear equalizer, and symbols are recovered from the compensated received signal with a detector, the method comprising: generating a control signal that is proportional to rotation of a vector formed by an In-phase component and a Quadrature component of an input signal of the detector, generating an additional signal that is proportional to a timing phase of the receiver symbol synchronization, wherein, at timings of said receiver symbol synchronization, said additional signal is not affected while compensating for timing phase drift in said received signal, and controlling the receiver symbol synchronization on the basis of a combination of said control signal and said additional signal.
 2. The method of claim 1, wherein the control signal that is proportional to the rotation of the vector formed by the In-phase and the Quadrature components of the detector input signal is obtained as a vector product of the detector input signal with a symbol decision signal.
 3. The method of claim 1, wherein the control signal that is proportional to the rotation of the vector formed by the In-phase and the Quadrature components of the detector input signal is obtained as a vector product of a detector error signal with a symbol decision signal.
 4. An apparatus for improving quality of receiver symbol synchronization on a digital communication connection for a receiver coupled to the digital communication connection where the receiver includes: means for receiving a signal passed over the digital communication connection the received signal being modulated with one of the following line codes: quadrature amplitude modulation and carrierless amplitude and phase modulation, an adaptive linear equalizer for compensating distortion to the received signal caused by the digital communication connection, and a detector for recovering symbols from the distortion compensated received signal, the apparatus comprising, in the receiver: means for generating a control signal that is proportional to rotation of a vector formed by an In-phase component and a Quadrature component of an input signal of the detector, means for generating an additional signal that is proportional to a timing phase of the receiver symbol synchronization, wherein, at timings of said receiver symbol synchronization, said additional signal is not affected while compensating for timing phase drift in said received signal, and means for controlling the receiver symbol synchronization on the basis of a combination of said control signal and said additional signal.
 5. The apparatus of claim 4, wherein the control signal is generated as a vector product of the detector input signal with a symbol decision signal.
 6. The apparatus of claim 4, wherein the control signal is generated as a vector product of a detector error signal with a symbol decision signal. 